Such a video server including video signal memories, a server control means, a broadband switch having first ports to which the memories are coupled, having second ports coupled to the communication network and having a third port to which the server control means is coupled for controlling the transfer of the video signals from the memories and through the switch and communication network to the terminals is known from the article "A system architecture for a large scale video on demand service" by W Sincoskie, Computer Networks and ISDN Systems 22, 1991, pp. 155-162. Therein the video memories comprise the "library", "copier memories" and "stop/start buffers". Although not described in the article, it is clear that for such a system to operate correctly, adaptor means have to be provided in order to adapt the video signals stored in the memories to a format compatible with the communication system and to provide them at a real time rate as needed for the terminals.
Indeed, the server has to provide video signals in real time and in a broadband format, e.g. an Asynchronous Transfer Mode (ATM) or a Synchronous Digital Hierarchy (SDH) or a Synchronous Optical Network (SONET) or a SONET format, although the video signals are stored in the video memories in a way which is independent from this broadband format and read therefrom at speeds that are higher than real time.
As a result of the described architecture these adaptor means have to be located at the outputs of the above memories and consequently their number has to be proportional to the number of video memories, and thus also of the number of available video signals, and this irrespective of the demand, i.e. of the number of adaptor means already in use.
In other words, when calling the resources, e.g. the adaptor means, needed to supply the specific video signals to a terminal, video channels, it is clear that the known video server has to allocate a number of video channels proportional to the number of video signals and this irrespective of the demand for these video signals. Normally, the number of video signals available can be much larger than the number of requested video signals or channel signals which may be supplied simultaneously. Hence it is clear that in the known server an excess number of video channels is allocated, which results in a waste of resources. However each of the video channels is needed to cater therefore the probability that an associated video signal, i.e. a video signal stored in the video memory corresponding to the video channel signal, is requested.
In the above, the drawbacks of the known server are related to resources. These resources can comprise hardware as well as bandwidth. Indeed, a specific number of channels requires the provision of e.g. a SONET link or other trunk line which has a specific bandwidth capability needed to be able to supply these channels. As a result, an excess of bandwidth is needed to provide the excess of channels.